For microwave systems in general tunable arrangements or components are of great importance. As examples of tunable arrangements can be mentioned resonators, filters, phase shifters and antennas. Particularly important are tunable components or arrangements for agile microwave systems. Typically the tunable components are implemented in the form of lumped inductors and capacitors (so called lumped LC devices) and sections of transmission lines where varactors (controllable capacitors) are used as tuning means. The varactors can be of many different kinds. For example micro-electro-mechanical varactors (MEM), alternatively semiconductor varactors, for example consisting of p-n junctions, MOS (Metal Oxide Semiconductors) varactors etc. The varactors may also be ferroelectric. Typically the varactors, the sections of the transmission lines and the LC devices are arranged as hybrid, monolithic integrated circuits wherein the lumped and distributed elements have microstrip, stripline or a coplanar structure. In order to increase the quality factor while still keeping the fabrication costs low, it has been suggested to use hollow waveguides as surface mounted components.
It is however a problem with such arrangements, comprising a tunable resonator and other integrated components which are based on lumped LC elements and sections of microstrip, coplanar waveguide and striplines, that they are associated with relatively high losses. This is mainly due to currents being highly concentrated in thin and narrow metal strips and since currents are concentrated in open structures which then will radiate. Even if surface mounted waveguides have smaller losses than other types of integrated waveguides, it is difficult to electronically tune the parameters of the electromagnetic waves travelling in such waveguides without a substantial reduction of the quality factor (Q-factor). It is also difficult to keep the fabrication costs low.